Freshwater mussels have declined rapidly in North America as a result of various factors including commercial exploitation, habitat degradation, impoundment and channelization, alterations of the natural flow regime, and the introduction of exotics. This study examines the status of a mussel assemblage located on the Marais des Cygnes River at the Marais des Cygnes National Wildlife Refuge in Kansas.
Four long-term monitoring locations were selected on the river and data were collected at the sites on several parameters that indicate mussel bed quality: density, species richness, extent of the bed, recruitment status, and habitat availability. A stratified, systematic sampling protocol was used which defined the boundaries of each mussel bed in the upstream, downstream, and cross-stream directions and concentrated sampling effort in areas close to the bank where mussels were heavily aggregated. Direct comparisons between mussel beds were made. Total density within the mussel bed at the four sites ranged from 3.0 mussels per m2 at Site 2 to 8.9 mussels per m2 at Site 4. The threeridge (Amblema plicata) was the most abundant species at all four sites with densities ranging from 1.3 mussels per m2 to 4.5 mussels per m2. At Sites 1–3 pimpleback (Quadrula pustulosa) was the next most abundant and washboard (Megalonaias nervosa) was second in abundance at Site 4. Although ranking third among the sites in total mussel abundance and area, the mussel bed at Site 1 had the highest value of species evenness, a greater amount of medium and coarse gravel, and higher numbers of recent recruits in four aged species than the other sites. The mussel bed extended 13 meters in the cross-stream direction. Site 3 and Site 4 consisted of narrow mussel beds that extended approximately 7–8 meters across the stream. Mussels were dense at these sites with the majority of individuals consisting of Amblema plicata, Quadrula pustulosa , or Megalonaias nervosa. The abundance of recent recruits was lower than the abundance of mussels greater than 15 year of age at these two sites. It is likely that abundance will gradually decline through the years as the older individuals die and are not replaced at three out of the four mussel beds examined.
By aging a collection of deceased stranded mussels at Site 1 and developing and applying growth models to the lengths of mussels collected during the survey, I determined that annual discharge during the breeding season did not have an impact on recent mussel recruitment of three tachytictic species at the site: Obliquaria reflexa, Quadrula pustulosa , and Amblema plicata. There was no statistical difference among years in recruitment success. In contrast a bradytictic species, Ellipsaria lineolata, experienced higher recruitment in years where mean monthly discharge in June through August was low in recent years. There was no evidence of E. lineolata recruitment in years when mean monthly discharge was high throughout these months. The timing coincides with the reported time of the year when gravid E. lineolata females release glochidia, infect an appropriate host, and metamorphosed juveniles excyst off of the host. The episodic recruitment of butterfly which is closely linked to long periods of lower than average flows during the breeding season, may partly explain the rarity of this species on the river.
Discriminant models were built on mussel microhabitat use at the four sites. Density classes could be distinguished from each other based on depth and substrate variables, but models based on these variables alone were limited in their ability to predict mussel presence or abundance in a given quadrat, particularly in habitats considered favorable. Mussels generally occupied areas with high amounts of medium and coarse gravel and low depth within a particular site but other factors including local shear velocity and bed stability at higher flows may be limiting their distribution. The local conditions at Site 1 have allowed at least three tachytictic mussel species to successfully recruit every year, while conditions at the three other sites are apparently unfavorable to mussel recruitment in recent years. (Abstract shortened by UMI.)